Method for production of a sintered component

ABSTRACT

The invention relates to a method for production of a sintered component (2), comprising the steps: making available an iron-based powder having chromium; filling the powder into a powder press; pressing the powder to form a green compact; sintering the green compact to form the sintered component (2); post-compacting the sintered component (2); hardening the sintered component (2). Sintering is carried out in a decarburizing atmosphere, and the sintered component (2) is moved, for surface-compacting, along an axis (3), from a first matrix opening (6) in the direction of a second matrix opening (13) of a matrix tool (1), which opening lies opposite the first matrix opening (6) along the axis (3), wherein the sintered component (2) passes through multiple matrix sections (7-11) of the matrix tool (1) during this movement, and, in this regard, a surface region of the sintered component (2) is compacted, for which purpose an inside diameter (17) of the matrix sections (7-11) that follow one another becomes smaller in the pressing direction, and the individual matrix sections (7-11) are disposed in such a manner that a subsequent matrix section (7-11) of the multiple matrix sections (7-11) directly follows the corresponding preceding matrix section (7-11), in each instance, in the pressing direction.

The invention relates to a method for the production of a sintered component, comprising the steps: making available an iron-based powder having chromium as an alloy element; filling the powder into a powder press; pressing the powder to form a green compact; removing the green compact from the powder press; sintering the green compact to form the sintered component; post-compacting the sintered component; hardening the sintered component.

Sintered components, in other words components that are produced according to a powder-metallurgy method, have the disadvantage, as compared with components produced from cast materials, that for many applications, the strength after sintering is not sufficient due to the porosity of the sintered component. In the state of the art, various methods have therefore already been proposed for post-compacting or surface-compacting of sintered components after sintering.

A usual method is rolling of rotation-symmetrical components, such as gear wheels, for example. As a representative of this, reference is made to WO 1992/005897 A1.

A method variant is compacting in a matrix tool, which is described in EP 2 066 468 A2. Similar methods are known from JP 10 085 995 A, AT 517 989 A1, and RU 2 156 179 C2.

Although these methods for post-compacting deliver good results, as such, problems occur during post-compacting of hard materials, such as iron powders that contain chromium, for example, as they are described in DE 10 2005 027 055 A1, for example, which deals with rolling of sintered gear wheels. For this purpose, the method according to DE 10 2005 027 055 A1 comprises the steps: filling a sintering material into a press, which has an inside geometry for forming a pre-form, wherein an allowance is formed at least in the region of one flank of the gearing; pressing the sintering material, so that a pre-form occurs; pre-sintering the pressed pre-form; surface-rolling at least one region of the flank of the gearing; sintering the component, sinter-hardening the component and precision finishing.

The task of the invention consists of creating a method with which hard sintering materials having a relatively high surface density can be produced.

This task is accomplished, in the case of the method stated initially, in that sintering is carried out in a decarburizing atmosphere, and that the sintered component is moved, for surface-compacting, along an axis, from a first matrix opening in the direction of a second matrix opening of a matrix tool, which opening lies opposite the first matrix opening along the axis, wherein the sintered component passes through multiple matrix sections of the matrix tool during this movement, and, in this regard, a surface region of the sintered component is compacted, for which purpose an inside diameter of the matrix sections that follow one another becomes smaller in the pressing direction, and the individual matrix sections are disposed in such a manner that a subsequent matrix section of the multiple matrix sections directly follows the corresponding preceding matrix section, in each instance, in the pressing direction.

It is advantageous, in this regard, that the hardness of the component is reduced by means of the decarburizing sintering, so that the subsequent compacting of the component can be carried out in a simpler and more efficient manner. Since the sintered component has been “clamped in” on all sides during post-compacting, very great surface densities can be achieved, even for iron materials that contain chromium, since the material cannot escape, as is the case during rolling of the gear wheels in accordance with the aforementioned DE-A1, in which the pressure is applied to the teeth of the gear wheel only radially. Furthermore, the method is not restricted solely to sintered components having rotation symmetry.

For better formability of the sintered components, it has proven to be advantageous, during the course of evaluation of the method, if at least one gas from a group consisting of oxygen, carbon dioxide, hydrogen is contained in the decarburizing atmosphere, wherein the proportion of the gas amounts to between 0.1 vol.-% and 10 vol.-% of the atmosphere.

Preferably, according to another embodiment variant of the method, the C proportion in the green compact being sintered is reduced by maximally 0.6 wt.-%. Surprisingly, it was possible to determine that this minimal reduction in the carbon content in the sintered component is already sufficient for improved formability if the surface-compacting is carried out in the said matrix tool.

According to a further embodiment variant of the method, it can be provided that after surface-compacting in the last matrix section having a decreasing inside diameter, relaxing of the sintered component is carried out in a stress-relief section that directly follows the last matrix section and has a greater inside diameter in comparison with the last matrix section configured directly ahead of it, the matrix section having a decreasing diameter, wherein the sintered component is calibrated in the stress-relief section, for which purpose the inside contour of this stress-relief section corresponds to the reference contour having a reference dimension of the sintered component. It is advantageous, in this regard, that before this calibration or intermediate calibration, no further forming of the sintered component from the stress-relieved state occurs, and thereby the ridge formation on the sintered component caused by the kneading effect during surface-compacting can be reduced. Furthermore, in this way the matrix tool is also subject to less mechanical stress, since further compacting of the sintered component from the stress-relieved state required greater forming forces, since this component was already surface-compacted in the preceding compacting steps.

Preferably, a powder is used that has a chromium proportion between 0.1 wt.-% and 10 wt.-%. In this way, sintered components having correspondingly good strength properties can be produced using the method, and thereby the field of use of sintered components can be broadened.

As explained above, hardening of the sintered component takes place after surface-compacting. In this regard, hardening preferably takes place by means of carburizing and subsequent quenching, or sinter-hardening and subsequent quenching, or inductive hardening.

In this regard, it can be provided that carburizing is carried out by means of low-pressure carburizing. In this way, the advantage can be achieved that even in the case of sintered components that are very narrow in the axial direction, hardness profiles can be set in a very targeted manner, in comparison with other carburizing methods, such as carbonitriding. It is therefore possible to obtain a softer core, even in the case of these sintered components.

According to a further embodiment variant of the method, quenching can be carried out using gas. By avoiding liquids for quenching, embedding of these liquids into the sintered component can be avoided, and thereby a very clean sintered component is already available according to the method.

For a better understanding of the invention, it will be explained in greater detail using the following figures.

The drawing shows, in a simplified, schematic representation:

FIG. 1 a section through a detail of a matrix tool for surface-compacting.

As an introduction, it should be stated that in the different embodiments described, the same parts are provided with the same reference symbols or the same component designations, wherein the disclosures contained in the description as a whole can be applied analogously to the same parts having the same reference symbols or the same component designations. Also, the position information selected in the description, such as at the top, on the bottom, on the side, etc., for example, relate to the FIGURE being directly described and shown, and this position information must be transferred analogously to a new position in the event of a change in position.

Production of metallic sintered components, such as gear wheels, for example, takes place according to a powder-metallurgy method (sintering method). Such methods are already very well known from the state of the art, so that a detailed discussion of the fundamentals of this method is unnecessary. In this regard, it only needs to be stated that the method Essentially comprises the Steps of making a powder available, filling the powder into a powder press, pressing the powder to produce a green compact, removing the green compact from the powder press, single-stage or multi-stage sintering of the green compact to produce the sintered component, post-compacting of the sintered component, and hardening of the sintered component. In the following, only the Essential Steps of the method according to the invention will therefore be explained in greater detail. With regard to the other method steps, reference is made to the relevant prior art.

The sintered component is produced from an iron-based powder with chromium as an alloy element.

According to a preferred embodiment variant, the sintered component or the powder has a proportion of chromium that is selected from the range from 0.1 wt.-% and 10 wt.-%.

The iron-based powder can have the following composition, for example: Fe+3% Cr+0.5% Mo+0.5% C or also Fe+1.8% Cr+2% Ni+0.5% C.

In general, the iron-based powder can contain the following components in the amount proportions indicated, along with chromium, wherein the proportions of the iron-based powder add up to 100 wt.-%, in each instance:

Fe: 90 wt.-% to 99.9 wt.-%

C: 0 wt.-% to 1 wt.-%

Mo: 0 wt.-% to 2 wt.-%

Ni: 0 wt.-% to 5 wt.-%

Cu: 0 wt.-% to 5 wt.-%

For the powder used, the pure elements or pre-alloys, if necessary with master alloys, can be used.

The powder is filled into the matrix of a powder press and pressed in it to form what is called a green compact, preferably coaxially pressed. The pressing pressure can amount to between 600 MPa to 1200 MPa, for example.

After removal of the green compact from the powder press, it is sintered to produce the sintered component. Sintering can take place in one stage, for example at a temperature between 900° C. and 1350° C., or in two stages, wherein in the first stage, the temperature can amount to between 800° C. and 1200° C., and in the second stage, it can amount to between 1100° C. and 1350° C.

Sintering (before post-compacting) is carried out in a decarburizing atmosphere. For this purpose, the sintering atmosphere can contain at least one gas from the group consisting of oxygen, carbon dioxide, hydrogen, and mixtures thereof. The proportion of the at least one gas in the decarburizing atmosphere can amount to between 0.1 vol.-% and 10 vol.-%. In the case of a mixture, the total proportion of the decarburizing gases can also amount to between 0.1 vol.-% and 10 vol.-%. The rest is formed by nitrogen and/or hydrogen, in each instance.

Preferably, according to one embodiment variant of the method, the proportion of the gas amounts to between 0.1 vol.-% and 2 vol.-% of the atmosphere.

In general, the carbon proportion of the sintering green compact can be reduced by 0.01 wt.-% to 0.8 wt.-% during sintering in the decarburizing atmosphere. According to a preferred embodiment variant of the method, however, the carbon proportion is only reduced by maximally 0.6 wt.-%.

Furthermore, according to another embodiment variant, it can be provided that the carbon proportion is reduced only in a surface layer having a layer thickness between 10 μm and 500 μm. This is achieved by means of targeted gas flow in the sintering oven.

Subsequent to sintering, the sintered component is post-compacted, wherein at least the surface and the region that follows it are compacted. The effect of the surface-compacting is greatest directly at the contact surface with the compacting tool, and decreases in the direction toward the interior of the sintered component. Using the method, edge layers of sintered components that contain chromium can be compacted at a thickness of a few hundredths of a millimeter up to multiple tenths of a millimeter and more.

A matrix tool 1, as shown in FIG. 1 in longitudinal section, using a preferred exemplary embodiment, is used for surface-compacting.

A sintered component 2, which is produced in accordance with the method steps mentioned above, is moved along an axis 3 by means of the matrix tool 1, for surface-compacting.

The matrix tool 1 comprises a basic tool body 4, which has a first (upper) matrix opening 6 on a tool surface 5, from which multiple matrix sections 7 to 11 lead into the interior of the basic tool body 4, along the axis 3. In this regard, the first matrix section 7 follows the first matrix opening 6; the last matrix section 11, in contrast, is located closer to a second tool surface 12 that lies opposite the first tool surface 5, along the axis, and a second matrix opening 13 formed in it.

In the exemplary embodiment shown, the sintered component 2 is structured in disk shape, for reasons of a better illustration, and has a diameter 15 on a radial outer surface 14, i.e. the face surface, which diameter corresponds to a raw diameter before compacting, and, after surface-compacting, corresponds to a final diameter that is smaller in comparison. However, the shape of the sintered component 2 as shown should not be understood to be restrictive.

Surface compacting of the sintered component 2 takes place in that it is introduced into the first matrix section 7 through the first matrix opening 6, and subsequently moved into all the further matrix sections 8 to 11, wherein in each matrix section 7 to 11, the outer surface 14 of the sintered component 2 is pressed against wall surfaces 16 of the matrix sections 7 to 11, at least at sections of the outer surface 14.

The pressing effect is achieved in that an inside diameter 17 of the matrix sections 7 to 11, which is defined by the clear width between opposite or interacting sections of the pressing surface of a matrix section 7 to 11, is smaller, in each instance, than the diameter 15 of the sintered component 2 before it is introduced into the respective matrix section 7 to 11. In general, the matrix sections 7 to 11 preferably have an inner contour that corresponds to the outer contour of the sintered component 2, wherein, however, each matrix section 7 to 11 has a circumference or a cross-sectional surface area that is smaller than the circumference or the cross-sectional surface area of the sintered component 2 before it is introduced into the respective matrix section 7 to 11.

The matrix sections 7 to 11, which follow one another along the axis 3, make a direct (constant) transition into one another, i.e. without intermediate sections, and have inside diameters 17 or cross-sectional surface areas that decrease (continuously) from the first matrix section 7 up to the last matrix section 11, in other words, matrix sections 7 to 11 become smaller but not larger. The movement of the sintered component 2 in the matrix tool 1 preferably takes place in a straight line in the pressing direction, from the first matrix opening 6 up to the last matrix section 11; subsequently, unmolding of the sintered component 2 from the matrix tool 1 preferably takes place after a reversal of direction counter to the pressing direction, through the first matrix opening 6.

The straight-line movement in the direction of the axis 3 can also have a rotational movement superimposed on it, and thereby the sintered component 2 performs a screw-type movement in the matrix tool 1.

The relative motion between the sintered component 2 and the matrix tool 1, which is required for carrying out the method, can take place by means of moving the sintered component 2 and/or by means of moving the matrix tool 1, wherein for this purpose, the sintered component 2 and the matrix tool 1 are each connected with a suitable drive or a fixed frame. During surface-compacting and subsequent calibration, the sintered component 2 is clamped in place between an upper punch 18 and a lower punch 19. For the downward movement, the upper punch 18 presses onto the sintered component 2 from above; in this regard, the lower punch 19 can be drawn downward, or it is also pressed downward by the upper punch 18. For the preferred ejection of the sintered component 2 by way of the first matrix opening 6, the lower punch 19 is pressed upward and, if necessary, the upper punch 18 can be pulled upward. Corresponding drives, not shown in detail, can be provided for these movements of the upper punch 18 and the lower punch 19.

The transition from one matrix section 7 to 10 to the subsequent matrix section 8 to 11 can be structured as a chamfer 20, or can be provided with a rounded part, wherein a concave rounded part can be followed by a convex rounded part in the pressing direction. In this way, a gentle transition of the sintered component 2 from one matrix section 7 to 10 to the subsequent matrix section 8 to 11 can take place, without unintentional material wear taking place on the sintered component 2 due to a sharp-edged step, or the edges breaking out at the transitions of the matrix tool 1. As is evident from FIG. 1, such a chamfer can also be formed on the first matrix opening 6. The chamfers 20 or the respective rounded parts are part of the respective matrix section 7 to 11, in other words do not form any intermediate sections.

Although five matrix sections 7 to 11 are shown in the embodiment variant of the matrix tool 1 shown in concrete terms in FIGS. 1 and 2, the matrix tool 1 can generally have between three and eight or more than eight such matrix sections.

The last matrix section 11 shown in FIG. 1 is the matrix section of the matrix tool 1 that has the smallest inside diameter 17 or the smallest clear width. Directly following this last matrix section 11 having the smallest inside diameter 17, a stress-relief section 21 can be provided or configured in the matrix tool 1, according to one embodiment variant of the method or of the matrix tool 1. This stress-relief section 21 has a greater inside diameter 22 in comparison with the last matrix section 11 having a decreasing inside diameter 17, which is configured directly ahead of it. As a result, the sintered component 2 can relax in this stress-relief section 21. At the same time with this relaxing, calibration of the sintered component 2 also takes place in the stress-relief section 21. For this purpose, the stress-relief section 21 has an inner contour that corresponds to the reference contour having a reference dimension of the sintered component 2. The inner contour of the stress-relief section 21 is therefore the same as the outer contour of the finished sintered component 2, both with regard to its geometry and with regard to the geometrical dimensions (viewed in cross-section).

At this point, it should be mentioned that calibration of a sintered component is understood to mean its processing to at least approximately produce the reference dimensions of the component in a tool, by means of pressing stress. In this regard, “at least approximately” means that deviations from the reference dimension, within the scope of the usual tolerances, are permissible.

The term reference dimension in the sense of the invention is understood to mean a final dimension that the finished sintered component 2 is supposed to have, if applicable minus the increase in size of the sintered component 2 after the relaxing that is defined by the spring-back behavior of the sintering material on the basis of elastic resilience. The proportion of the spring-back behavior can be determined empirically. Stated in different words, the reference dimension plus any increase in size that occurs as the result of the elastic resilience results in the final dimension.

Subsequent to the stress-relief section 21, the matrix tool 1 preferably also has a further section 23. This section 23 has an inside diameter 17 or a clear width that corresponds to the inside diameter 17 or the clear width of the last matrix section 11 having the smallest inside diameter 17. The section 23 serves to guide the lower punch 19 in the matrix tool 1.

The inside diameter 22 or the clear width of the stress-relief section 21 corresponds to the outside diameter 15 (FIG. 1) or the clear width of the finished sintered component 2. This inside diameter 22 or this clear width of the stress-relief section 21 is greater by at least 0.02%, in particular between 0.02% and 0.1%, than the inside diameter 17 or the clear width of the last matrix section 11 having the smallest inside diameter 17. However, the inside diameter 22 or the clear width of the stress-relief section 21 is not greater than the inside diameter or the clear width of the first matrix opening 6. In this way, at least approximately complete relaxing of the sintered component 2 is supposed to be made possible.

According to an embodiment variant of the method for surface-compacting of the sintered component 2, it can be provided that the penultimate matrix section 10, viewed in cross-section, is configured identically with the cross-section of the stress-relief section 21 and thereby with the calibration cross-section, both with regard to its geometry and also the geometrical dimensions in cross-section.

After surface-compacting and, if applicable, calibration of the sintered component 2, it is hardened. Fundamentally, any hardening method suitable for this purpose and known from the state of the art can be used.

According to a preferred embodiment variant of the method, however, hardening takes place by means of carburizing and subsequent quenching, or by means of sinter-hardening and subsequent quenching, or by means of inductive hardening.

The carbon proportion in the sintered component 2 is increased by means of carburizing. Carburizing can fundamentally take place by means of different methods, wherein all the methods have in common that a gas or a gas mixture is used as a carbon source. Methane, propane, acetylene, etc., for example, can be used as a gas. Carburizing can be carried out in a further sintering step, for example subsequent to surface-compacting. Carburizing can also take place by means of carbonitriding. Preferably, however, carburizing takes place by means of a low-pressure carburizing method.

After carburizing, the carbon content of the sintered component 2 preferably amounts to between 0.1 wt.-% and 1.0 wt.-%.

In particular, carburizing can be carried out to a depth of the sintered component 2, measured from its surface, which is selected from a range from 100 μm to 2000 μm; preferably from a range from 100 μm to 1000 μm. The preferred content of carbon mentioned above relates to the carburizing depth, in this regard. Regions of the sintered component 2 that lie underneath can accordingly have a lower carbon content.

Subsequent to carburizing, the sintered component is quenched. Quenching can also take place using a suitable method known from the state of the art, for example by means of oil quenching. Preferably, however, quenching of the sintered component 2 is carried out using a gas, for example using N₂, N₂/H₂ or He. The quenching speed can be selected from a range from 1° C./s to 7° C./s.

It can also be provided that a powder that can be hardened by means of sintering is used for production of the green compact. This is understood to mean an iron powder or steel powder that has a proportion of at least one alloy element that delays eutectoid conversion of austenite to ferrite and pearlite. For example, the powder can have a proportion of nickel and/or molybdenum in addition to chromium. The proportion of at least one alloy element in the powder for production of the green compact can amount to between 0.4 wt.-% and 5 wt.-%.

After sinter-hardening, the sintered component is also quenched. According to another embodiment variant of the method, it can be provided that the sintered component is carburized again. Preferably, this step of carburizing takes place simultaneously with the second sintering step, if sintering is carried out in two stages, as described above. For carburizing (recarburizing), a carburizing gas, such as methane or propane, for example, can be added to the sintering atmosphere.

However, carburizing can also be carried out using another known carburizing method.

By means of carburizing, the carbon content of the sintered component can again be increased by 0.1 wt.-% to 1.0 wt.-%.

If necessary, mechanical post-processing can take place after hardening.

Using the process route described, it is possible to produce sintered components 2 having little distortion.

The exemplary embodiments describe possible embodiment variants, wherein combinations of the individual embodiment variants with one another are possible.

For the sake of good order, it should be pointed out, in conclusion, that for a better understanding of the structure, the matrix tool 1 is not necessarily shown to scale.

REFERENCE SYMBOL LIST

-   1 matrix tool -   2 sintered component -   3 axis -   4 basic tool body -   5 tool surface -   6 matrix opening -   7 matrix section -   8 matrix section -   9 matrix section -   10 matrix section -   11 matrix section -   12 tool surface -   13 matrix opening -   14 outer surface -   15 diameter -   16 wall surfaces -   17 inside diameter -   18 upper punch -   19 lower punch -   20 chamfer -   21 stress-relief section -   22 inside diameter -   23 section 

1: A method for the production of a sintered component (2), comprising the steps: providing an iron-based powder having chromium; filling the powder into a powder press; pressing the powder to form a green compact; removing the green compact from the powder press; sintering the green compact to form the sintered component (2); post-compacting the sintered component (2); hardening the sintered component (2); wherein sintering is carried out in a decarburizing atmosphere, and wherein the sintered component (2) is moved, for surface-compacting, along an axis (3), from a first matrix opening (6) in the direction of a second matrix opening (13) of a matrix tool (1), which second opening lies opposite the first matrix opening (6) along the axis (3), wherein the sintered component (2) passes through multiple matrix sections (7-11) of the matrix tool (1) during this movement, and, in this regard, a surface region of the sintered component (2) is compacted, for which purpose an inside diameter (17) of the matrix sections (7-11) that follow one another becomes smaller in the pressing direction, and the individual matrix sections (7-11) are disposed in such a manner that a subsequent matrix section (7-11) of the multiple matrix sections (7-11) directly follows the corresponding preceding matrix section (7-11), in each instance, in the pressing direction. 2: The method according to claim 1, wherein at least one gas from a group consisting of oxygen, carbon dioxide, hydrogen is contained in the decarburizing atmosphere, wherein the proportion of the gas amounts to between 0.1 vol.-% and 10 vol.-% of the atmosphere. 3: The method according to claim 1, wherein the C proportion in the sintering green compact is reduced by maximally 0.6 wt.-%. 4: The method according to claim 1, wherein after surface-compacting in the last matrix section (11) having a decreasing inside diameter (17), relaxing of the sintered component (2) is carried out in a stress-relief section (21) that directly follows the last matrix section (11), which stress-relief section (21) has a greater inside diameter (22) in comparison with the last matrix section (11) configured directly ahead of it of the matrix section (7-11) having a decreasing inside diameter (17), wherein the sintered component (2) is calibrated in the stress-relief section (21), for which purpose the inner contour of this stress-relief section (21) corresponds to the reference contour having a reference dimension of the sintered component (2). 5: The method according to claim 1, wherein the chromium proportion of the powder amounts to between 0.1 wt.-% and 10 wt.-%. 6: The method according to claim 1, wherein hardening takes place by means of carburizing and subsequent quenching or sinter-hardening and subsequent quenching or inductive hardening. 7: The method according to claim 6, wherein carburizing is carried out by means of low-pressure carburizing. 8: The method according to claim 6, wherein quenching is carried out using a gas. 